1. Field of the Invention
The invention relates to nanoparticles and methods for making nanoparticles, and in particular, to semiconductor nanocrystals that exhibit improved hydrophilicity.
2. Discussion of Related Art
Nanoparticles are microscopic particles of matter having dimensions on the nanometer scale. Of particular interest are a class of nanoparticles known as semiconductor nanocrystals, or quantum dots, that exhibit properties that make them particularly useful in a variety of applications. Because of quantum confinement effects, semiconductor nanocrystals can exhibit size-dependent optical properties. The particles give rise to a class of materials whose properties include those of both molecular and bulk forms of matter. When these nanoparticles are irradiated, more energy is required to promote the electrons to a higher state, leading to an increase in energy release in the form of photons and light emission in a color that is characteristic of the material. The resulting photons that are released typically exhibit a shorter wavelength than those released from a bulk form of the same material. The quantum confinement of electrons and holes in three dimensions contributes to an increasing effective band gap with decreasing nanocrystal size. Therefore, smaller nanocrystals typically exhibit shorter emitted photon wavelength. For example, nanocrystals of cadmium selenide (CdSe) can emit across the entire visible spectrum when the size of the crystal is varied over the range of from two to six nanometers.
Another aspect of semiconductor nanocrystals is that crystals of a uniform size typically are capable of a narrow and symmetric emission spectrum regardless of excitation wavelength. Thus, if nanocrystals of different sizes are employed, different emission colors may be simultaneously obtained from a common excitation source. These capabilities contribute to the nanocrystals' potential as diagnostic tools, for example, as fluorescent probes in biological labeling and diagnostics. These nanocrystals, or quantum dots, exhibit high emission stability over long periods of time, thus providing advantages over conventional biological probing dyes. One class of semiconductor nanocrystals are the cadmium chalcogenides. These include, for example, cadmium selenide and cadmium telluride nanoparticles.
It is known that improved quantum yields of semiconductor nanocrystals can be obtained by passivating the nanocrystals by reducing the incident of surface non-radiative recombination sites. Surface passivation can be achieved, for example, by coating a material around the nanocrystals. See, e.g., Alivisatos et al., U.S. Pat. No. 6,255,198. The coatings can be inorganic or organic although inorganically coated quantum dots are typically more robust and exhibit less degradation of photo luminescence quantum yield in solution than do organically passivated quantum dots.
For semiconductor nanocrystals to be useful in biological applications, it is preferred that the crystals are water soluble, photo-stable and non-toxic. Some quantum dots may exhibit water solubility but are typically not photo-stable and are toxic. Other nanocrystals have been coated, for example, with short chain water soluble molecules, such as thiols, to render the nanocrystals soluble. However, these organically coated quantum dots have been shown to be unstable and exhibit deteriorating photo-luminescent properties. Others, such as Bawendi et al. in U.S. Pat. Nos. 6,319,426 and 6,444,143, hereby incorporated by reference, have synthesized semiconductive nanocrystals having an organic layer that also includes linking groups for the attachment of hydrophilic groups that can provide improved water solubility.
Some have proposed coating nanocrystals using silicate as a precursor. These methods use silane as a surface primer to deposit a thin shell of silica in water. The silica shell can then be thickened using the Stöber method. These procedures, however, are complicated and time-consuming. Others have used microemulsions as a technique for silica coating. In particular, using reverse microemulsions, monodispersed silica particles can be synthesized. Encapsulation of nanoparticles within silica can lead to an enhancement in chemical stability and photo-stability. This has been done in nanoparticles having a zinc sulfide (ZnS) core/two photon dye/silica particles and the encapsulated dye within the silica shell has exhibited enhanced luminescence and lifetime. However, synthesized TOPO semiconductor nanocrystals are water insoluble and thus silica cannot be precipitated with the nanocrystals within the aqueous domains of the microemulsion.